Silicon substrates of various types are used in numerous applications including solar cells and display devices. So-called amorphous silicon (a-Si) is used in high resolution liquid crystal displays to provide an active layer in which thin film transistors can be defined. Amorphous silicon can be deposited in thin films using PECVD. Low temperature polysilicon (LTPS) can be produced by exposing an a-Si layer to high intensity ultraviolet laser pulses that rapidly melt the deposited a-Si layer without heating the underlying substrate. The a-Si layer then crystallizes in grains having sizes that are dependent upon temperature gradients during processing. Typical mobility of an LTPS layer is on the order of about 50-150 cm2/V-sec, superior to the 0.5 cm2/V-sec mobility associated with a-Si.
Conventional LTPS processing is based on surface treatment with an excimer laser, or so-called excimer layer annealing (ELA). In ELA, a line-shaped, approximately uniform laser beam (typically at 308 nm) is directed as a series of pulses of durations of about 25 ns to an a-Si layer that is heated and melted. The molten layer then recrystallizes to form a layer of polycrystalline silicon (p-Si). Laser pulse energy and beam uniformity must be precisely controlled. Each area of the target a-Si layer is exposed to several excimer laser pulses, and the heating, melting, and recrystallization process also repeats. The resulting LTPS layer exhibits a rectangular array of crystalline regions as shown in FIGS. 1A-1B. Processing is generally targeted to produce crystalline regions or “grains” that have dimensions of about 300 nm for most thin film transistor (TFT) backplanes.
Excimer lasers are complex and expensive to maintain as production equipment. Even the best excimer lasers tend to have very limited service lifetimes, and replacement of excimer laser cavities and associated optical components thereof can be disruptive and expensive. Although satisfactory results can be obtained, total processing costs associated with ELA remain high.